From: "Tom Krajci" 
To: , ,
        ,

Reply-To: "Tom Krajci" 


The Foucalt knife edge test is often used with a Couder mask to help
isolate the zones being tested.

Unfortunately for the outer zones...the tester must decide if the two
widely spaced zones are equal in brightness.  This is playing against an
inherent weakness of the eye...accurate comparison of brightness of widely
separated objects.  (This is one reason some folks favor wire and caustic
tests.  The eye measures positions of shadows...which the eye can do pretty
well.)

OK, plenty of folks still insist on testing with a Couder mask.  For those
folks I have a question:

If one were to set up a light source with adjustable brightness to shine on
the Couder mask...and adjust it so that it were equal in brightness (or
close to it) to the outer zones being tested...would this help increase
accuracy/sensitivity of the human eye in our effort to match the brightness
of the two widely separate zones?  (And perhaps this is a worthwhile
technique for testing all zones...not just the outer zones?)

It does allow another variable to enter into the testing procedure...is
this light that shines on the Couder mask vignetted in any way...or is part
of the Couder mask dirty/smudged in one part?  If this variable is not
controlled it would put a systematic error in the test results. However, if
controlled carefully, maybe this technique can help work around one problem
with Foucalt/Couder zonal testing.

I'm sure others have tried this before.  What are their findings?  Any
formal studies been done?  Where can I find them?

Thanks in advance,
Tom Krajci
Tashkent, Uzbekistan


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